Customers Repairs: Hacker Radios

Hacker Sovereign RP18, Repair D018

 Diagnosis: The receiver was reasonable on medium and long waves although it took a minute or so to stabilise. It wasn't working at all on the FM band and there was also a short in the tuning condenser over part of the AM bands.

 Repair: The basic problem is short circuiting within the transistors used in this model. The old AF117 and AF114 germanium transistors suffer a crystalline growth on their substrates which short-circuits the collector to ground. The resistance of the short varies and is often not serious enough to stop the particular transistor from completely ceasing to pass a signal. Battery consumption in this state is very high and is often the reason for people stopping using the radio.
I opened up the VHF tuner module and found these transistors to be AF121 and an AF125 which do not suffer from the same shorting problem. There are 4 IF amplifier transistors in the FM strip and 3 in the separate AM strip. I cut the connection between the case/substrate and ground in each of these transistors. This is a recognised expedient solution to the problem.
I then realigned the FM front end and the associated 10.7MHz IF amplifier. The VHF receiver now performs very well, albeit over the restricted tuning range comensurate with the vintage of the receiver. The AM receiver also performed well after resetting its oscillator so that the tuning scale read correctly and dealing with the short in the tuning condenser which I removed by slightly bending the vanes then adjusting the rear screw to centre the moving vanes.


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